Hitherto, polyurethane foam, being widely used in various fields and appreciated of its special quality, shows however a gross weak point in its flame-resistance. And yet, the control of flame-resistance of polyurethane foam materials is tending lately to become severe more and more and this situation has brought out a very important problem of how to technically make polyurethane foam flame resistant.
As regards how to make polyurethane foam flame resistant, many kinds of technique have already been proposed. However, the most preferably used one of these techniques is a method of adding, as flame retardants, a phosphorous and halogen containing compound such as tris (.beta.-chloroethyl) phosphate. In the case of adopting this method, however, it is necessary for conferring flame-resistance on polyurethane foam to add such a great amount of phosphorus containing compound as to make the rate of phosphorus content in the foam higher than about 1%. Thus, it is necessary to add an expensive phosphorus, and halogen containing flame retardants in such a quantity as to amount 10 to 30 parts by weight per 100 parts by weight of polyhydroxyl compound, the polyurethane foam produced proves consequently to be of fairly high cost. Saying about the physical properties of the foam, further, the plasticizing effect of the added phosphorus and halogen containing flame retardants causes the polyurethane foam to be reduced in hardness, decreased in compressive strength and increased in density so that the foam is markedly lowered in the physical properties of matter. Besides, as said conventional type of flame retardant tends, when used at an elevated temperature, to undergo volatilization loss with the passage of time so as to be weakened in its effect, there arising thus a problem of suffering a fatal decrease in the flame resistance. Lately moreover, as it was confirmed an accident due to corrosion progress in a heavy oil tank equipped with a heat insulation construction made up of rigid polyurethane foam that the accident took its rise in a phosphorus and halogen containing flame retardant as one of compounding ingredients of the foam, the suspensive using of conventional flame retardants has created a remarkable stir in the field of construction works of heat insulation.
Since various questions are involved as above-mentioned in using phosphorus and halogen containing compounds as flame retardant, the development of a technique has already been strongly demanded which confers flame resistance to polyurethane foam without using any phosphorus and halogen containing compounds.
The present inventors, concentrating their energies on investigation for replying to the above-mentioned requirement, arrived finally at the discovery of a surprising fact that inexpensive melamine powder can be utilized quite with effect to confer flame resistance on polyurethane foam. The usage of triazine ring containing compounds as one of compounding ingredients for polyurethane foam has already been proposed by way of, for example, Japanese patent laid open No. 141650/75, Japanese patent laid open No. 84844/76 and Japanese patent laid open No. 16461/76. While these well-known arts make use of a triazine ring containing compound as an ingredient of foam to improve the fatigue bearability of polyurethane foam against repeated stress thereon, it has hitherto been quite unknown the fact that melamine powder has an excellent flame resistant effect on polyurethane foam. The present inventors, being based upon this information, added melamine powder to the conventional formulation of rigid polyurethane foam instead of using a phosphorus and halogen containing compounds as flame retardants and thereby succeeded in manufacturing rigid polyurethane foams which have an excellent flame resistance and show only small decrease in the physical properties; their investigation led to the present invention.